1. Field of the Invention
The present invention generally relates to a rolling bearing assembly used in, for example, a multipurpose drive motor, an electric power generator and a traction motor for electric railway cars and, more particularly, to the rolling bearing assembly having an improved resistance to electric corrosion, which can be suitably used in an environment where an electric current flows.
2. Description of the Prior Art
As is well known to those skilled in the art, the rolling bearing assembly employed in a traction motor for electric railway cars is generally so designed that, in the event of troubles occurring in an earthed collector for grounding an electric current from the railway traction motor to the rails through wheels, the electric current flowing in the railway traction motor flows through inner and outer races and rolling elements of the rolling bearing assembly before it is grounded to the rails. Accordingly, sparking takes place between the rolling elements and raceway grooves in one or both of the outer and inner races, accompanied by an electric corrosion of the component parts of the rolling bearing assembly. Once this occurs, the lifetime of the rolling bearing assembly is shortened.
In order to minimize the electric corrosion, it has been suggested to form an electric insulating layer of a synthetic resin on a surface area of the outer race, which is held in contact with a housing for supporting the rolling bearing assembly. However, the electric insulating layer made of a synthetic resin has so large a coefficient of linear thermal expansion that an error tends to occur in fit between the outer race and the housing by the effect of heat evolved by the rolling bearing assembly during the operation of the rolling bearing assembly.
In view of the above, for material of the electric insulating layer, a ceramic material is considered feasible because of its low coefficient of linear thermal expansion and high electric insulating property and, in fact, the Japanese Laid-open Utility Model Publication No. 2-46119 and the Japanese Laid-open Patent Publication No. 2002-48145, for example, disclose a thermal spraying in which a ceramic material is sprayed to form a ceramic layer as an electric insulating layer. The thermally sprayed insulating layer has a film thickness so chosen as to secure a required electric insulating property that can be satisfied with a commercially available high insulating material.
Alumina (Al2O3) and gray alumina (Al2O3+TiO2) are well known as a ceramic material that can be used to form a thermally sprayed ceramic layer as an electric insulating layer. Comparing those ceramic materials with each other, they have the following representative characteristics, which differ as follows:                Volumetric Resistivity White Alumina>Gray Alumina        Dielectric breakdown voltage White Alumina>Gray Alumina        Yield at Thermal Spraying White Alumina<Gray Alumina        
Since gray alumina contain TiO2, which is an electroconductive substance, the volumetric resistivity of the gray alumina tends to become lower than that of white alumina. The greater the content of TiO2 in the gray alumina, the lower the volumetric resistivity thereof. Accordingly, the white alumina is rather feasible if a high electric insulation performance is of prime importance.
It has been empirically known that for a given film thickness the white alumina exhibits a higher dielectric breakdown voltage than that exhibited by the gray alumina and, therefore, the white alumina is considered feasible where a high dielectric breakdown voltage is of prime importance.
The white alumina has a melting point higher than 2,000° C. whereas the TiO2 has a melting point lower than 2,000° C. Accordingly, the gray alumina containing TiO2, when thermally sprayed, exhibits a high deposit efficiency and, as such, the white alumina, when compared with the gray alumina, does not bring about a high deposit efficiency and, hence, a poor yield of deposition. The term “deposit efficiency” referred to above and hereinafter stands for what fraction of the amount of the spraying material used to thermally spray on a component to be coated is actually deposited on the component to form the coating or layer. Thus, it will readily be seen that, in order to secure a predetermined film thickness, the gray alumina capable of bringing about a good yield of deposition during the thermal spraying can be considered feasible.
The volumetric resistivity and measurements conducted in the past to determine and the dielectric breakdown voltage have made it clear that the white alumina has an electric insulation performance superior to that exhibited by the gray alumina, but as far as the deposit efficiency during the thermal spraying is concerned, the white alumina is rather inferior to the gray alumina.
As discussed above, as a material for the electric insulating layer to be employed in the rolling bearing assembly, the white alumina and the gray alumina have their own merits and demerits in respect of their performance and moldability and, therefore, unless the proper material is chosen, the rolling bearing assembly would provide no proper performance of resistance to electric corrosion and require complicated and time-consuming assemblage, accompanied by increase of the cost.